Background Most mental disorders start in childhood and adolescence. Risk factors are prenatal and perinatal, genetic as well as environmental and family related. Research evidence is, however, insufficient to explain the life-course development of mental disorders. This study aims to provide evidence on factors affecting mental health in childhood and adolescence.

Data and methods The 1987 Finnish Birth Cohort covers all children born in Finland in 1987 (N=59 476) who were followed up until the age of 21 years. The study covers detailed health, social welfare and sociodemographic data of the cohort members and their parents from Finnish registers.

Results Altogether, 7578 (12.7%) cohort members had had a diagnosed mental disorder. Several prenatal, perinatal and family-related risk factors for mental disorders were found, with sex differences. The main risk factors for mental disorders were having a young mother (OR 1.30 (1.16 to 1.47)), parents’ divorce (OR 1.33 (1.26 to1.41)), death of a parent (OR 1.27 (1.16 to 1.38)), parents’ short education (OR 1.23(1.09 to 1.38)), childhood family receiving social assistance (OR 1.61 (1.52 to 1.71)) or having a parent treated at specialised psychiatric care (OR 1.47 (1.39 to 1.55)). Perinatal problem (OR 1.11 (1.01 to 1.22)) and prenatal smoking (OR 1.09 (1.02 to 1.16)) were risk factors for mental disorders, even after controlling for background factors. Elevated risk was seen if the cohort member had only basic education (OR 3.37 (3.14 to 3.62)) or had received social assistance (OR 2.45 (2.30 to 2.60)).

Conclusions Mental disorders had many social risk factors which are interlinked. Although family difficulties increased the risk for mental disorders, they were clearly determined by the cohort member's low education and financial hardship. This study provides evidence for comprehensive preventative and supporting efforts. Families with social adversities and with parental mental health problems should be supported to secure children's development.